E-MBMS is an evolution of multi-media broadcast and multicast service in 3GPP LIE. MBMS transmissions may be performed in following two ways: Multi-cell transmissions and Single-cell transmissions. The present invention relates to a single-cell transmission mode.
In case of single-cell transmission, i.e. a cell specific to multipoint transmission, a content of multicast service is independent in each cell. The MBMS data from multiple cells need not synchronously be transmitted. In 3GPP LIE specification, an MTCH (MBMS Traffic Channel) will be mapped to a DL-SCH (Downlink Synchronization Channel).
In E-UTRNA, DL-SCH is mainly mapped by a dedicated traffic channel (DTCH). To improve a peak rate of UE unicast traffic and a system throughput, DL-SCH is characterized by using HARQ (High Automatic Repeat Request), adaptive modulation coding and transmit power, dynamic or semi-static resource allocation, CQI reporting and etc.
When the MTCH is mapped to the DL-SCH, this part of the DL-SCH should be broadcasted in the entire cell. To provide a required data rate and BLER (Block Error Ratio) at the edge of cells, similar procedures as used by the unicast traffic for the DL-SCH are envisaged, such as HARQ.
Similar to HSDPA (High Speed Downlink Packet Access), a physical HARQ can decrease the BLER in a receiver so as to achieve higher transmission performance, but a tradeoff is decreasing of transmission efficiency. From the experience of HSDPA deployment, the first transmission's BLER is between 10%˜30%, which could achieve an optimal performance-efficiency ratio.
However, when the HARQ is applied in multicast systems, since there are multiple UEs receiving the downlink multicast traffic simultaneously, in principle only one UE received an error block and returned a NACK signaling, a base station (eNB) should retransmit this data block.
From a point view of probability theory, if n UEs are located in one cell, the error probability is α for all the UEs in a first transmission, then the probability that at least one UE received the error block is 1−(1−α)n. If the number of the UEs is large, this probability would be very high. Therefore some mechanisms should be applied to restrict the feedback of the NACK signaling, or to trigger the retransmission of data blocks.
A target is to firstly restrict that the UE whose BLER is less than 30% may feed back the NACK, secondly to limit the probability of retransmission in eNB, for example 50%. In addition, if the UE feeds back too much NACK, the uplink would be overload. Thus it should restrict the number of NACK signaling upon the UEs receive the error block.
The present invention relates to applying HARQ in a method for triggering a retransmission in a multicast system, which is independent to the way of the UE feeding back the NACK signaling and the way of eNB retransmitting.